Growth, Differentiation and Sexuality

336 S. Pöggeler et al.
restricted than that controlling vegetative growth
(Moore-Landecker 1992), but the genetic basis for
this is not yet clear. mod-E, a heat-shock protein
HSP90homolog,wasfoundtobeinvolvedinboth
sexual development and vegetative incompatibility
in Podospora anserina (Loubradou et al. 1997).
mod-E transcripts are accumulated after a shift
from 26 to 37 ◦ C, but effects of different temperatures
on fruiting-body formation in the wild
type versus mod-E mutants were not reported.
Therefore, it remains to be determined whether
mod-E or other (heat-shock) proteins are involved
in temperature-dependence of fruiting-body
development.
B. Endogenous Factors
The transition from vegetative growth to sexual development
requires a physiologically “competent”
mycelium. This competence often depends on nutrient
availability, but the nutrients also have to be
processed by the fungal metabolism; and genetic
analyses have shown that fruiting-body formation
requires metabolic reactions different from those
of vegetative growth (see Sect. II.B.1). In several
fungal species, pheromones or hormone-like substances
are necessary for completion of the sexual
cycle, as described in Sect. II.B.2 and in Chap. 11
(this volume).
1. Metabolic Processes
In several ascomycetes, it was found that mutations
in genes for primary metabolism often interfere
with sexual development under conditions where
vegetative growth remains more or less normal.
Examples for this are mutants blocked in amino
acid biosynthesis pathways, and fatty acid biosynthesis
mutants. Many effects of mutations leading
to amino acid auxotrophy on fruiting-body morphogenesis
have been investigated in A. nidulans.
Deletion of the tryptophan synthase-encoding
gene trpB, or the histidine biosynthesis gene
hisB leads to loss of cleistothecia production
on medium with low levels of tryptophan or
histidine, respectively (Eckert et al. 1999, 2000;
Busch et al. 2001). Both genes are regulated by
the cross-pathway control system, a regulatory
network that activates a variety of amino acid
biosynthesis genes when the amounts of a single
amino acid are low. Besides regulating amino
acid biosynthesis, this cross-pathway network
also comprises a control point for progression of
sexual development (Hoffmann et al. 2000). This
was demonstrated by investigating the functions
in fruiting-body formation of two members of
the cross-pathway network, cpcA and cpcB. cpcA
encodes a transcriptional activator homologous to
the yeast Gcn4p protein, which is the activating
transcription factor for cross-pathway control
(termed general control of amino acid biosynthesis)
in Saccharomyces cerevisiae (Hoffmann
et al. 2001b). CpcB is homologous to mammalian
RACK1 (receptor for activated C-kinase 1), a scaffold
protein involved in many cellular signaling
processes (McCahill et al. 2002). cpcA and cpcB
play antagonistic roles in cross-pathway control
as well as in sexual development: cpcA activates
amino acid biosynthesis gene transcription under
conditions of amino acid deprivation, whereas
cpcB represses the cross-pathway control network
when amino acids are present. Overexpression
of cpcA in the presence of amino acids leads to
a block in sexual development, thereby mimicking
a lack of amino acids, and the same effect can be
reached by deletion of cpcB (Hoffmann et al. 2000).
The connection between cross-pathway control
and sexual development seems to be widespread
in filamentous ascomycetes, as a mutant in the
N. crassa cpcB homolog, cpc-2, is female-sterile
(Müller et al. 1995). Also, a sterile mutant of
S. macrospora was shown to have a defect in
a gene for leucine biosynthesis (Kück 2005). This
mutant, as well as the A. nidulans amino acid
biosynthesis mutants mentioned above, grow
normally on media with moderate amounts of
the amino acid they are auxotrophic for, but if at
all, fertility can be restored only by much higher
amounts. These findings indicate that fungi are
able to integrate nutrient availability and cellular
metabolism, and react properly with respect to the
initiation of energy-demanding processes such as
fruiting-body formation.
Similar regulatory events can be proposed for
fatty acid metabolism and fruiting-body development,
although the evidence here is more spurious
and signal transduction pathways have yet to
be identified. Nevertheless, data from mutants in
diverse genes involved in different aspects of fatty
acid metabolism indicate that appropriate amounts
and composition of fatty acids and their derivatives
are essential for sexual development. N. crassa
mutants of a fatty acid synthase subunit are sterile
in homozygous crosses, and A. nidulans mutants
of several desaturase genes show changes in